AS1312 datasheet(8/20 Pages) AMSCO

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AS1312

Datasheet - D e t a i l e d D e s c r i p t i o n

Figure 10. Simplified Voltage and Current Waveforms

Another important relationship is the “volt-seconds” law. Expressed as following:

(EQ 4)

Voltages are those measured across the inductor during each time segment. Figure 10 shows this graphically with the shaded segments marked

“A & B”. Re-arranging (EQ 4):

(EQ 5)

The time segment called TWAIT in Figure 10 is a measure of the “hold-up” time of the output capacitor. While the output voltage is above the

threshold (0.99xVOUT), the output is assumed to be in regulation and no further switching occurs.

8.1.2

Inductor Choice Example

For the AS1312 VIN_MIN = 0.9V, VOUT_MAX = 3.3V, (EQ 5) gives Ton=2.66TOFF.

Let the maximum operating on-time = 1µs.

Note that this is shorter than the minimum limit on-time of 3.6µs. Therefore from (EQ 5), TOFF = 0.376µs. Using (EQ 3), LMAX is obtained:

LMAX = 1.875µH. The nearest preferred value is 2.2µH.

This value provides the maximum energy storage for the chosen fixed on-time limit at the minimum VIN.

IPK

VOUT

VIN

TWAIT

TOFF

TON

TWAIT

TOFF

SW1_on

SW2_off

SW1_off

SW2_on

VIND_TON

VIND_TOFF

VOUT Ripple

0.99VOUT_NOM

ON TO N

O FFTO FF

O N

O FF

-------------

O UT

–

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